Three-microemulsion synthesis

In a later work [222], a unique three-microemulsion system was put to use for synthesis. The reverse micellar solution had DODMAC (surfactant) and 1-decanol (co-surfactant) in isooctane. DODMAC (= dioctyldimethylammonium chloride) was added via Bardac-LF-80 containing about 80% DODMAC in ethanol/water... 

Figure 1. TEM images of Pd/Ti02 catalysts prepared by the microemulsion method under three different synthesis conditions.

Physical-chemical studies require traces of additives (reactants, catalysts, electrolytes) with respect to the concentration of the basic components of the microemulsion, and this causes only a minor change in the phase behavior of the system. However, when the amounts of additives are on the scale used in organic synthesis, the phase behavior, which is very sensitive to the concentration of the reactants, is sometimes difficult to control and the reaction is carried out in a one-, two- or three-phase state. 

Three main methods are available for microemulsion-mediated materials synthesis, i.e., microemulsion-plus-trigger, microemulsion-plus-microemulsion, and microe-mulsion-plus-reactant (l 4,16). It can be seen from Table 2.2.1 that silica has been prepared by the microemulsion-plus-reactant (MPR) protocol exclusively that is, the oil-soluble alkoxide is added to the microemulsion. Referring to Table 2.2.1, four... 

The rate constant at the interface (kA) can be obtained as the slope of the straight line by plotting A ma(l — a) (1 — y )2 versus a (Fig. 5.3). This approach was used for describing the kinetics of the synthesis of 1-phenoxyoctane from sodium phenoxide and 1-bromooctane in a microemulsion based on the non-ionic surfactant Triton X-100, which is an octylphenol ethoxylate [27]. The total interfacial area was calculated from known values of the head group area of the non-ionic surfactant. As shown in Fig. 5.3, straight lines were obtained from which the rate constants could be obtained. From the values of kA determined at the three different temperatures, an activation energy of 85 kj mol-1 was calculated. This is a typical value for an SN2 reaction, as usually determined in homogeneous reaction media. 

In this chapter, we have demonstrated the potential of surfactant-based reaction media for preparative organic chemistry. We have not attempted to give a complete account of all reactions that have been investigated in microemulsions and related media. Instead, we have chosen to concentrate on three important and illustrative examples nucleophilic substitutions, regioselective synthesis and hydrogenation and hydroformulation reactions. We believe that taken together they provide a good picture ofwhat surfactant-based reaction media can offer to synthetic chemistry. 

Microemulsion-mediated materials synthesis employs three basic methods, as illustrated in Fig. 3 [17]. The microemulsion-plus-trigger method (Fig. 3, method a) is based on a single microemulsion. The fluid system is activated in some way in order to initiate the reactions that eventually lead to particle formation. Pulse radiolysis and laser photolysis have served as triggers for the preparation of nanosize gold particles [41]. In the case of metal oxides, temperature elevation can provide the needed trigger action hydrated metal ions solubilized... 

A brief summary of the information available till around 1990 [211] shows the use of three reverse microemulsion systems for the synthesis of (amorphous) silica particles AOT /isooctane /water, AOT /benzyl alcohol/ decane /water and NP-5 /cyclohexane/ water. Note that the dispersed water phase had a dissolved base (NH4OH) or acid (HCl) as catalyst in it. Tetraethyl orthosilicate (TEOS) was added to the reverse microemulsions, leading to hydrolysis-condensation reaction and formation of silica particles. The size of the particles depended on the experimental conditions, but could go down to about 15 nm. 

Polystyrene Nanoparticles. Pol3mier-based nanoparticles are used in several areas of life science, for example drug delivery (56), and one such poljoner matrix commonly used is poly(L-lactic acid) (57). However, nanoparticles with a polystyrene-DVB matrix have been also synthetized (58,59) and used as novel solid supports for organic synthesis (60). The method commonly used to generate these monodisperse particles is Microemulsion Polymerization (qv) (58). However, nanoparticles have also been prepared by precipitation polymerization (61). A microemulsion of styrene and DVB, with an amphiphilic comonomer, in water (three component oil-in-water) (62) allows the preparation of nanobeads with a hard core of polystyrene and the amphiphihc comonomer dispersed on the surface with diameters around 50 and 300 nm that can be precipitated by the addition of methanol. Functionalization of the nanoparticles surface, can be easily achieved using the functionalized amphiphihc comonomer (Fig. 9) (60). 

Protease-catalyzed synthesis of two precursor dipeptides of the hydrophilic tripeptide RGD (Arg-Gly-Asp) in three different types of w/o microemulsions, namely AOT/isooctane, Triton/ethyl acetate, and CTAB/heptane/hexanol, was reported by Chen and coworkers [59]. RGD is a tripeptide of pharmaceutical interest consisting of one neutral (Gly) and two hydrophilic amino acids (Arg and Asp). Among the proteases tested, alcalase and trypsin were selected as the most efficient catalysts and the reverse micellar system consisting of... 

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